1. Field of the Invention
The present invention relates to an optical information reproducing apparatus which reproduces recorded information from an optical recording medium.
2. Description of the Related Art
An optical disk used as an optical recording medium includes a transparent substrate of a predetermined thickness for covering and protecting the recording surface of the disk. An optical pickup irradiates a reading light beam on the recording surface through the transparent substrate. The optical pickup reads recorded information from the optical disk based on a reflected light from the recording surface.
However, it is difficult to form the thicknesses of the transparent substrates of all optical disks to be of a predetermined value during manufacture of the disks, and typically, a thickness error of several tens xcexcm arise. As a result, spherical aberration is generated by the thickness error in the transparent substrate. When the spherical aberration is generated, the amplitude level of a tracking error signal can be significantly reduced, leading to the problem that the information reading accuracy is reduced.
Accordingly, it is an object of the present invention to provide an optical information reproducing apparatus capable of conducting accurate reading of information even if spherical aberration is generated by the thickness error in the transparent substrate of an optical disk.
An optical information reproducing apparatus according to the present invention which reproduces recorded information from an optical recording medium, comprises an optical system including a laser generating device for generating a laser light beam, an objective lens for condensing said laser light beam onto a recording track formed on the optical recording medium and a photo detector for photoelectric converting reflected light from the optical record medium into a photoelectric converting signal; a tracking servo circuit for controlling the laser light beam condensed with the objective lens so as to trace the recording track; a spherical aberration correcting part for correcting a spherical aberration of the optical system in accordance with a correction amount; a tracking servo gain measuring circuit for measuring a tracking servo gain in the tracking servo circuit; and aspherical aberration correcting controller changing the correction amount so as to the tracking servo gain becomes maximum.